Stimulation of production well for in situ metal mining

ABSTRACT

Metal values are economically leached in situ by rubblizing a portion of an ore body, injecting a lixiviant for the metal values through one or more injection wells in the ore body located adjacent to but outside the rubblized portion of the ore body, and collecting the lixiviant containing the dissolved metal values from one or more production wells located in a rubblized zone of the ore body.

United States Patent 1191 Girard et a1.

[ 1 Oct. 15, 1974 [54] STIMULATION OF PRODUCTION WELL 3,490,534 1/1970Grady 166/271 FOR IN s METAL MINING 3,542,131 11/1970 Walton et a1,166/299 X [75] Inventors: Lucien Girard, Boxboro; Robert A. PrimaryExaminer Ernest R Purser H d, L

ar exmgton both of Mass Attorney, Agent, or FzrmLowell H. McCarter; John[73] Assignee: Kennecott Copper Corporation, L. Sniado New York, N.Y.

[22] Filed: Sept. 27, 1973 57 ABSTRACT .Appl. 401,484 Metal values areeconomically leached in situ by rubblizing a portion of an ore body,injecting a lixivi- 52 US. 01 299/4, 166/247 166/271 am for the metalvalues through one or more injection 51 1111. C1 E2111 43/28 Wells inthe Ore body located adjacent to but Outside [58] Field of Search 299/4;166/247 271, 299 the rubblized portion of the ore body, and collectingthe lixiviant containing the dissolved metal values 5 References Citedfrom one or more production wells located in a rub- UNITED STATESPATENTS bl1zed zone of the ore body. 3,278,233 10/1966 Hurd et a1 299/410 Claims, 2 Drawing Figures PREGNANT LEACHING LEACHING SO E C INGSOLVENT SOLVENT PAIENIEBQU 1 51974 n 0 mo 0 W0 0 I O 0 0 o o o I 0 a 0 OO O I. I O O O 0 yo 9 0 B// O O O a 0 FIGURE 1 LEACHING SOLVENT OQOQ GOO

PREGNANT LEACHING SOLVENT ll LEACHING SOLVENT FIGURE 2 FIELD OFINVENTION In situ mining of valuable minerals has been explored as ameans of economically recovering the metal values from low grade ores orotherwise inaccessible ore bodies. Recent work in the area has, ingeneral, dealt with pumping of a leaching liquid or lixiviant into anore formation, allowing sufficient residence time and removing thepregnant liquid from the formation. Many problems have surfaced withrespect to the movement of fluids in the subsurface ore bodies and someinvestigation has been accomplished. U.S. Pat. Nos. 3,278,233 and3,574,599 are cited as showing the present state of in situ leachingart.

Other recent investigatory work has been done in the area of nuclearexplosive applications in forming lixiviation cavities in subsurfaceformations. Very recent art in this area, however, teaches the useof theexplosive to create a closed system or series of closed systems in theore body in which to introduce the lixiviant. Adelmann, Canadian Pat.No. 855,525 and Lewis, U.S. Pat. No. 3,640,579, demonstrate this use ofthe nuclear blast. This use of a closed system may well lead toefficient leaching within the rubblized zone but as Lewis indicates, theblast area is confined and, therefore, so is the leaching liquid. Thequantity of ore which can be thus contacted and leached is therebylimited to the rubblized region. The present invention proposes toincrease the volume of ore which canbe contacted and leachedeffectively,- without increasing the rubblized region.

SUMMARY The invention is a process of in situ mining comprising thesteps of rubblizing a portion of an ore body by detonating one or morestrategically placed explosive devices to form a rubblized zone orchimney, injecting a leach solution into one or more injection wells,the injection well or wells being located in the ore body adjacent tobut outside of the rubblized chimney, such that the leach solution willpass through the nonrubblized ore body removing the metal valuestherefrom and into the rubblized chimney, and recovering a pregnantleach solution from one or more production wells located within therubblized zone or chimney.

In an alternate method an ore body is solution mined in-situ by firstinjecting a lixiviant into the ore formation under pressure through anarray of injection wells. The pregnant solution is then recovered fromthe formation through one or more production wells that has beenstimulated with a nuclear or chemical explosion subsequent to theinjection of the leaching solution. The explosive charge causesrubblization in the area of the production well, but not in theimmediate area of the injection wells, such that the rubblizationreduces the resistance to lixiviant flow into the production well to theextent that the pregnant solution can be withdrawn from the productionwell nearly as fast as leaching liquid can be injected into the oreformation.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 shows one possible array for wellplacement, commonly known as the 5-spot pattern.

FIG. 2 shows the rubblization around the centrally located productionwell and the flow of leaching solvent from the distant injection wellsinto the rubblized zone.

DESCRIPTION OF INVENTION In situ solution mining of a metal valuerequires injecwith the mineral containing the metal values to beextracted, and recovery of the pregnant solution from the ore formation.The problem to be overcome in the process is the restricted flow ratesof leaching solution between the injection and the production wells,especially in igneous rock systems, due to the relatively lowpermeability of the igneous rocks. This restriction to fluid flow meansthat the leaching solution can be injected into the formation fasterthan it can be withdrawn.

The above situation is termed a production limited system becauseoverall recovery of pregnant solution, and therefore recovery of leachedmetallic element, is dependent on how fast solution can be withdrawnfrom the production well. The present invention is anattempt to reach aninjection limited system such that recovery of pregnant solution islimited by the amount of solution which can be injected into theformation. This situation would typically result in much greaterproductivity than the production limited system.

Referring now to FIG. 1, the ore body 13 is located and an array ofinjection wells 12 and production wells 11 are bored. The particularpattern shown in FIG. 1 is commonly known as a S-spot pattern whichallows the nearest neighbor of any injection well to be a productionwell and vice versa. It is within the scope of the invention describedherein that any other desired pattern of wells may be used.

In a preferred embodiment all the production wells 11 will be rubblizedwith a chemical or nuclear explosive device to provide a rubblized zone14 around each production well. The area of rubblization 14 indicatesthe relative extent of the blast area with respect to the placement ofthe injection wells. It will be noticed here,

and in greater detail in FIG. 2, that there remains relativelyundisturbed regions between the rubblized area and the injection well.The arrows around the rubblized area 14 indicate the direction of fluidflow from nearby injection wells. The injection wells may be betweenabout feet up to about 700 feet from the boundaries of the rubblizedchimney area 14. The distance between production and injection wellswilldepend on permeability profile of the rock mass surrounding thechimney.

The rubblized area surrounding the production wells 11 creates a oflower permeability thus resulting in the flow of fluid from the highpermeability igneous rock surrounding the injection wells 12 toward theproduction wells 11 rather than a fluid flow into other areas of the orebody-where the fluid may be lost.

From FIG. 2 it can be seen that the rubblized zone 2, causedby thenuclear or chemical explosion, does not extend to the injection wells 4and 5. However the nuclear chimney from the blast may extend upward fromthe ore formation 1 to a point above the water table 6. In this respectthe invention relates to the recovery of metal values found in mineralformations located beneath the ,water table and, hence, the rubblizedzone of the production well will be below the water table. This fact, inturn, means that the hydrostatic pressures beneath the water tablecontribute integrally to the flow properties of leaching solution intothe production well.

If desired it is possible to have more than one explosive detonation.For example, rubblized chimneys of desired geometric configurations canbe formed by strategically placing and detonating the explosives inproper sequence. See U.S. Pat. No. 3,470,953. Explosive reactionmixtures may be formed in concentric regions in the ore body around thewellbore by alternately injecting a propping agent and then an explosivereaction mixture. Upon detonation of the concentric regions of explosivereaction mixtures, the resulting fractures propagate both outwardly intothe ore body and inwardly toward the wellbore so as to enhance thesubsequent recovery of metal values from both the fractured areas andthe areas between the injection well or wells and the centrally locatedproduction well. For further details, see U.S. Pat. No. 3,593,793.

The present invention of an in situ leaching process has particularapplication to copper ore deposits,for example chalcopyrite, chalcocite,covellite and bornite. However the process is not limited to recoveringcopper but can be used in recovering many other metallic elements fromtheir ores. Of interest would be nickel, zinc, molybdenum, silver, goldand other valuable metals.

With respect to recovery of copper from primary sulfide deposits, theleaching solvent injected into the formation would typically be'water,sulfuric acid, and ferrous sulfate, in various proportions, togetherwith an oxidizing agent. The oxidizing agent is preferably anoxygen-bearing gas such as air, oxygen and any and all mixtures thereof.The oxygen-bearing gas may be introduced into the ore body prior to,during, or subsequent to the leaching solution. On contact with theleaching solution the copper is leached from the ore as a sulfate, andis later recovered from the solution by electrolysis, or in aprecipitation process using de-tinned iron to replace the copper insolution.

More particularly, for instance, when a soluble substance such as copperis to be recovered from an ore, which contains either metallic copper orcopper oxide in igneous rock, the present invention provides for veryefficient in situ leaching by an appropriate acid or alkaline liquidextractant, such as aqueous sulfuric acid of moderate strength, e.g.,acid containing 0.5 to 30 percent, and preferably 2 to percent, H 80 byweight. Such sulfuric acid is pumped into the injection well in thecopper ore in quantity sufficient to provide therein about 20 to 80pounds H 50 per ton of ore to be treated when such ore contains betweenabout 0.5 and '2 percent copper by weight. The pressures developed inpumping the leach solution into the ore body may vary over a wide range.Preferably the injection pressure will be below the formation fracturepressure. Recovery of copper from the formation by such leaching shouldbe between about 40 and 85 percent or more, e.g. 70 percent somewhatdepending on proportion and strength of acid used, permeability of theformation, concentration of copper therein, etc.

When the metal is in the ore as a sulfide, arsenide, telluride or asulfo-salt, it can be recovered from such a sulfide-type ore either byconverting it by in situ oxidation to the oxide by blowing airunderground and then extracting the oxide as described above.

If nuclear stimulation of the production well is selected as therubblizing agent, the nuclear explosives useable may range in yield fromas little as 10 kiloton up to 2,000 kilotons, it being noted that thecost of such explosives is relatively independent of the energy yield.Consequently, the largest size device that can be employed at the givendepth appropriate for a particular job is generally the most economicalone.

The following examples illustrate the invention with reference to coppercontaining ore bodies. It is to be understood that this is done solelyby way of example and is intended neither to delineate the scope of theinvention nor limit the ambit of the appended claims, since theinvention is applicable to the in situ recovery of other lixiviantsoluble metal values from ore bodies.

EXAMPLE I An embodiment of the invention as applied to the recovery ofcopper from a formation which contains copper oxide will next bedescribed. The Copper ore deposit to be treated is a stratum 1,000 feetthick and has 2,000 feet of overburden above it. This stratum con tainscopper in a concentration of about 1 percent in igneous gangue. At thebeginning of the operation a 50 kiloton thermo-nuclear explosive isplaced in the formation through a well at a depth of 3,200 feet. Whenthe well is sealed and the device detonated, a chimney approximately 650feet high is created above the shot point. It is estimated that thechimney will have a diameter of about 265 feet and contain about 2million tons of rubble. If it is found that additional copper ore needbe caved down, this may be produced by placing or detonating a furthernuclear device in the formation.

Aqueous sulfuric acid having an H content of about 5 weight per cent isthen pumped into the injection wells located some distance outside therubblized chimney. Sufficient volume of acid of proper strength shouldbe introduced into the injection wells to allow for the ultimateconsumption of about 40 pounds H 80 per ton of broken rock treated inthe process. After the acid is introduced in the injection wells it willpercolate through the ore body and take some copper oxide into solution,which will accumulate as a body of liquid in the lower part of thechimney.

After a period of time the accumulation of pregnant leach solution inthe chimney cavity can be pumped out through the production well locatedin the chimney. The copper is then recovered from the pregnant leachsolution by known methods.

EXAMPLE ll An ore body acres in area and averaging 100 feet in thicknesslies at an average depth of 3,000 feet below the surface of the earth.Samples of the ore shows that it is composed primarily of graniticigenous rock and that it contains chalcopyrite as the principle coppermineral. The ore samples also show that it contains approximately 1.4weight percent chalcopyrite and that the total copper content of the oreaverages 0.5 percent. The volume of ore in the deposit is, therefore, 10acre-feet, or 4.356 X 10 cubic feet. The specific gravity of thegranitic ore is 2.6. Therefore the total weight of the ore in thedeposit is 3.54 X 10 tons, and the copper content of the ore body is3.54 X 10 pounds.

Wells are drilled into the body in an array such that the well densityis one per acre. Position of production wells is determined. Liquidslurry or nuclear explosives are strategically placed and detonated suchthat the injection well positions remain outside of the rubblizedchimney area. By measurements on core samples and by injection andproduction tests on individual wells, it is determined that the voidvolume within the randomly oriented fracture system is equivalent to 2percent of the bulk ore volume, that the fracture spacing averages 6inches, and that the permeability of the ore body to liquid averagesabout 25 millidarcys.

Petrographic examination of core samples taken from the ore body showsthat about 2 percent of the rock surface area exposed by the fracturesis covered by the chalcopyrite mineral and that the rock matrix boundedby the fracture system is substantially cubical in configuration.

Thus the surfaceto-volume ratio of the ore blocks bounded by thefractures is approximately equal to that for cubically shaped blocks,and the surface area to volume ratio for the ore blocks is equal to 6/L,where L is the length of the side of a cube. In this case L 0.5 feet,and the surface area to volume ratio is equal to 12 square feet/cubicfoot.

The total surface area of ore exposed by the fracture network is equalto 12 X 4.356 X or 5.227 X 10 square feet. The surface area of thechalcopyrite mineral exposed by the fracture system is equal to 2percent of the total surface area, or 1.045 X 10 square feet.

Laboratory tests with the ore samples showed that ferric sulfatesolutions will dissolve copper from the chalcopyrite of the ore body ata rate equal to 0.002 pound of copper per square foot of chalcopyritesurface area per day. The initial maximum rate of copper productionattainable from the ore body by in situ leaching with ferric sulfatewould be 0.002 X l.045 X 10 209,000 pounds of copper per day. Thelaboratory tests also showed that by allowing a 0.4 molar solution offerric sulfate to react completely with the chalcopyrite, and otherminerals in the ore, a pregnant leach solution containing 3.0 pounds ofcopper per barrel (42 gallons) could be obtained. Therefore in order tosupply 0.4 molar ferric sulfate solution to the ore body at the optimumrate, i.e., at the rate sufficient to produce the maximum amount ofcopper and at the same time allow total reaction of the ferric iron, the0.4 molar ferric sulfate solution must be injected initially at a rateof 69,700 barrels/day. The required average residence time for thesolution within the ore body is fixed by the injection rate and the voidvolume of the ore body:

Average Residence Time void volume/injection rate (0.02) (4.34 X 10cubic feet/(69,700 bbl./day) (5.615 cu.ft./bbl.) 22.2 days The injectionand withdrawal rates of the wells are thus regulated to permit theferric sulfate solution to remain in the ore body for approximately 22days.

This average residence time, or the average time required for the fluidto traverse the ore body between injection and production wells, must beincreased as the chalcopyrite mineral is depleted and the surface areaof chalcopyrite exposed to the leaching solution diminishes. Over theuseful life of the in situ leaching operation, the optimum averageresidence time for the 0.4 molar ferric sulfate solution will becontinuously increasing and may be substantially greater than the 22.2days calculated as the optimum average residence time at the start ofthe operation.

In most cases, the injection and production rates should beapproximately equal in order to minimize migration of fluids into oraway from the ore body being subjected to the solution mining process.In this example if half of the wells are used as injection wells, andthe other half of the wells are used as production wells, the averageinjection and production rates will be initially:

69,700 barrels/day/SO wells 1,394 barrels/(day)(well) The injection andproduction rates at individual wells may be varied as necessary tomaintain an approximate overall balance between total injection andtotal production, and to maintain the residence time required foressentially complete reaction of the ferric iron in the leachingsolution with the ore minerals.

As noted above, it will be necessary to adjust the residence time of theleaching solution within the ore body to maintain the optimum residencetime as the ore minerals are depleted. The need for such adjustment willbe indicated by the appearance of ferric iron in increasingconcentrations in the fluids produced from the production wells. Whenferric iron is observed in the fluid produced from a production well,the rate of fluid withdrawal from that well should be adjusted untilferric iron is no longer found in the fluid produced from the well. Theinjection rates at nearby injection wells should then be correspondinglyreduced to maintain an overall balance between injection and production.This operation should be repeated as necessary to maintain the optimumresidence time for the leaching fluid.

We claim:

1. An improved process for recovering metal values by in-situ leachingan ore body located below the water table which comprises:

a. forming a rubblized zone in an ore body whereby the rubblized zonecontains fractured metal bearing ore particles;

b. injecting a leach solution through one or more injection wellslocated in the ore body adjacent to but outside the rubblized zone, theleach solution solubilizing metal values in the ore body and in therubblized zone; and

c. recovering a metal containing leach solution through one or moreproduction wells located in the rubblized zone.

2. The process of claim 1 wherein the rubblized zone is produced bydetonating one or more strategically placed explosives in the ore body,said explosive selected from nuclear and chemical explosives.

3. The process of claim 1 wherein the one or more injection wells arelocated between at least about 50 feet and up to about 700 feet from theone or more production wells.

4. The process of claim 1 wherein the ore body contains a copper bearingore.

5. The process of claim 4 wherein the leach solution is injected throughthe one or more injection wells at a pressure less than the formationfracture pressure.

6. The process of claim 5 wherein the leach solution contains adispersion of an oxygen bearing gas.

from at least one production well located in a rubblized zone in the orebody.

8. The process of claim 7 wherein the ore body contains copper bearingore.

9. The process of claim 8 wherein the leach solution is aqueous sulfuricacid containing an oxygen bearing gas.

10. The process of claim 9 wherein at least one injection well islocated between about 50 feet and about 700 feet from at least oneproduction well.

d UNi'iED STA'EES PATENT GFFICE (IRTIFICATE 9i QQRRECTWN Patent No.3,841,705 Dated October 15 1974 mventofls) .Lucien Girard et a1 It iscertified that error appears in the aboveidentified patent and that saidLetters Patent are hereby corrected as shown below:

Columnl; line 53, "a of lower" should read a zone of higher line 54,"high" should read l w Column 4,

line 57, "shows" should 'read show line 58, "igenous" should readigneous Signed and sealed this 18th day of February 1975.

(SEAL) Attest 2 c. MARSHALL DANN RUTH C2o MASGN Commissioner of PatentsAttesting Officer and Trademarks FORM PO-105O (IO-69) Notice of AdverseDecision in Interference In Interference No. 99,293, involvin and R. A.Hard, STIMULATION OF PRODUCTION SITU METAL MINING, final to thepatentees was rendered Jan. 10, 1980, as to Claims 1, 2, 49.

[Oficz'al Gazette, A pm'l 29, 1980.]

1. AN IMPROVED PROCESS FOR RECOVERING METAL VALUES BY INSITU LEACHING ANORE BODY LOCATED BELOW THE WATER TABLE WHICH COMPRISES: A. FORMING ARUBBLIZED ZONE IN AN ORE BODY WHEREBY THE RUBBLIZED ZONE CONTAINSFRACTURED METAL BEARING ORE PARTICLES; B. INJECTING A LEACH SOLUTIONTHROUGH ONE OR MORE INJECTION WELLS LOCATED IN THE ORE BODY ADJACENT TOBUT OUTSIDE THE RUBBLIZED ZONE, THE LEACH SOLUTION SOLUBILIZING METALVALUES IN THE ORE BODY AND IN THE RUBBLIZED ZONE; AND C. RECOVERING AMETAL CONTAINING LEACH SOLUTION THROUGH ONE OR MORE PRODUCTION WELLSLOCATED IN THE RUBBLIZED ZONE.
 2. The process of claim 1 wherein therubblized zone is produced by detonating one or more strategicallyplaced explosives in the ore body, said explosive selected from nuclearand chemical explosives.
 3. The process of claim 1 wherein the one ormore injection wells are located between at least about 50 feet and upto about 700 feet from the one or more production wells.
 4. The processof claim 1 wherein the ore body contains a copper bearing ore.
 5. Theprocess of claim 4 wherein the leach solution is injected through theone or more injection wells at a pressure less than the formationfracture pressure.
 6. The process of claim 5 wherein the leach solutioncontains a dispersion of an oxygen bearing gas.
 7. The method ofleaching a metal bearing ore in place which comprises: a. injecting aleach solution at a pressure below the formation fracturing pressurethrough at least one injection well located in a non-rubblized zone of ametal bearing ore body located below the water table; b. allowing theleach solution to remain in the ore body to solubilize metallic ionspresent in the ore body, and c. recovering metallic ion containing leachsolution from at least one production well located in a rubblized zonein the ore body.
 8. The process of claim 7 wherein the ore body containscopper bearing ore.
 9. The process of claim 8 wherein the leach solutionis aqueous sulfuric acid containing an oxygen bearing gas.
 10. Theprocess of claim 9 wherein at least one injection well is locatedbetween about 50 feet and about 700 feet from at least one productionwell.